Tight junctions seal epithelial cells together to create a permeability barrier between the lumenal cavity and the interstitial space of epithelial tissues including liver. Hepatocytes form an epithelium that transports electrolytes and other solutes into the biliary tree and maintains a barrier between plasma and bile. Alterations of this barrier may be involved in the pathogenesis of certain gastrointestinal diseases, such as ulceration of the gastric mucosa, Crohn's disease, or cholestasis. The permeability properties of the tight junction are regulated by hormones, but nothing is known about the biochemical mechanisms of their regulation. The A6 cell line, derived from the kidney of Xenopus laevis, provides an excellent model epithelium with which to investigate the biochemical mechanisms of tight junction assembly and regulation. Glucocorticoid hormones specifically induce the development of high transepithelial electrical resistance in these cells, and several immunological and biochemical tools are available to study proteins of their junctions. The major hypothesis to be tested in this proposed research is that the induction of high transepithelial electrical resistance in the A6 cell line by glucocorticoid hormones is due to specific molecular modifications of intercellular junctions. The overall objectives of the research plan are 1) to identify changes in known junctional proteins that correlate with the induction of transepithelial electrical resistance, and 2) to attempt to exploit this selective induction of tight junctions to identify novel protein components of the tight junction. Since glucocorticoids often act to increase the transcription of specific genes, experiments will be done to determine whether junctional proteins, including E-cadherin, its associated catenin polypeptides, ZO-1, vinculin, alpha-actinin, and desmoplakin are selectively induced. Also, the induction of high transepithelial resistance by glucocorticoids will be exploited to attempt to identify novel proteins associated with the tight junction. These studies should contribute to our understanding of the basic biochemistry of tight junction regulation, and may provide new approaches to explore possible tight junction defects in liver disease and other gastrointestinal diseases.